The kokanee salmon (Oncorhynchus nerka), also known as the kokanee trout, little redfish, silver trout, kikanning, Kennerly's salmon, Kennerly's trout, or Walla, is the non-anadromous form of the sockeye salmon (meaning that they do not migrate to the sea, instead living out their entire lives in freshwater). There is some debate as to whether the kokanee and its sea-going relative are separate species; geographic isolation, failure to interbreed, and genetic distinction point toward a recent divergence in the history of the two groups. The divergence most likely occurred around 15,000 years ago when a large ice melt created a series of freshwater lakes and rivers across the northern part of North America. While some members of the salmon and trout family (salmonids) went out to sea (anadromous), others stayed behind in fresh water (non-anadromous). The separation of the sockeye and the kokanee created a unique example of sympatric speciation that is relatively new in evolutionary terms. While they occupy the same areas and habitats during the breeding season, when ocean-going sockeye salmon return to freshwater to spawn, the two populations do not mate with each other in some regions, suggesting speciation.
Etymology
The name kokanee means "red fish" in the Sinixt Interior Salish language, and "silver trout" in the Okanagan language.
Distribution
Kokanee are native to many lakes in the western United States and Canada including Alaska, Washington, Oregon, California and Idaho in the United States and British Columbia and Yukon in Canada. Populations of kokanee are also found in Japan and Russia. Additionally, kokanee have been introduced to many other lakes in the United States including in those states mentioned above as well as in Montana, Wyoming, Nevada, Utah, Colorado, Arizona and New Mexico in the west, as well as in New England, New York and North Carolina in the east. Kokanee have also been introduced to lakes in southern Alberta and Saskatchewan in Canada.
Genetic and morphological diversity
There is debate as to whether the kokanee has enough genetic distinction to be classified as a subspecies or a separate species from the sockeye salmon. Genetic evidence suggests that the evolution of landlocked sockeye has occurred more than once with different kokanee populations, showing genetic differences between populations. Most evidence points to events that land-locked the lake-type sockeye (which spawns in streams and lakes, rather than tributaries like the sea-type sockeye, but is still anadromous and spends most of its adult life out at sea), which evolved into the non-anadromous form. Genetic evidence from kokanee in the Fraser River drainage and Columbia River drainage shows that ancestors of the kokanee came from the lake-type sockeye. The data also suggests that the kokanee may have evolved back into a lake-type anadromous form at some point in recent history, although there is a lack of interbreeding between it and sockeye in the drainage systems. It is also important to note that genetic distinction between sockeye and kokanee that cohabitate varies from region to region, with some populations showing distinct divergence, but others showing very little divergence. Studies done in Okanagan Lake in British Columbia and Lake Sammamish in Washington State suggested that the genetic diversity between the lake-type sockeye and the kokanee marks the divergence of two species because cohabitating sockeye and kokanee did not interbreed, despite the fact that interbreeding was possible.
There is some morphological divergence between the kokanee and sockeye. The most noticeable difference is size. The kokanee is often smaller than the sockeye due to less nutrient availability in freshwater. Other morphological differences include higher gill raker counts in the kokanee, failure to develop the changes that allow the sockeye to transition between fresh and salt water, and juvenile kokanee's lower mean swim velocity compared to juvenile sockeye. Egg sizes are also a difference. Due to its size, the kokanee should theoretically have smaller eggs in order to increase fecundity. In reality, the kokanee egg size varies, even though the energetic cost of larger eggs can limit the fecundity of kokanee populations. Studies have found that kokanees can extract carotenoids (which provide red pigmentation during breeding) from food better than sockeyes due to sexual selection and mate choice. Again, the degree of morphological variation, such as gill raker count, can vary from population to population.